Linearized spectrum correlation analysis for thermal helium beam diagnostics.

T Nishizawa,P Manz,S Kado,D Wendler,R Dux,G Grenfell,M Cavedon,M Griener

doi:10.1063/5.0062436

Abstract

We introduce a new correlation analysis technique for thermal helium beam (THB) diagnostics. Instead of directly evaluating line ratios from fluctuating time series, we apply arithmetic operations to all available He I lines and construct time series with desired dependencies on the plasma parameters. By cross-correlating those quantities and by evaluating ensemble averages, uncorrelated noise contributions can be removed. Through the synthetic data analysis, we demonstrate that the proposed analysis technique is capable of providing the power spectral densities of meaningful plasma parameters, such as the electron density and the electron temperature, even under low-photon-count conditions. In addition, we have applied this analysis technique to the experimental THB data obtained at the ASDEX Upgrade tokamak and successfully resolved the electron density and temperature fluctuations up to 90 kHz in a reactor relevant high power scenario.

Highlights

A thermal helium beam (THB) diagnostic is capable of measuring the electron density ne and the electron temperature Te in the scrape-off layer (SOL) and the confined region near the last closed flux surface (LCFS) in hot magnetized plasmas.[1,2,3,4] In this diagnostic technique, a neutral helium gas is injected into the plasma, and the active emissions of multiple He I lines are observed
We have introduced linearized spectral spectrum correlation analysis (LSCA) for the thermal helium beam diagnostic
The time series with the desired parameter dependence can be expressed as a linear function of the electron density and the electron temperature around its mean values

Summary

INTRODUCTION

A thermal helium beam (THB) diagnostic is capable of measuring the electron density ne and the electron temperature Te in the scrape-off layer (SOL) and the confined region near the last closed flux surface (LCFS) in hot magnetized plasmas.[1,2,3,4] In this diagnostic technique, a neutral helium gas is injected into the plasma, and the active emissions of multiple He I lines are observed. While GPI is able to resolve fast time scale fluctuations up to several hundred kHz, interpreting data is not straightforward because the active line emission intensity depends on multiple parameters.[9]. Linearized spectral correlation analysis (LSCA)[13,14] overcomes a similar issue in ion Doppler spectroscopy, whose time resolution is typically limited by observable light intensities. The principle of LSCA is based on the ensemble average and cross correlation We reformulate LSCA for the line-ratio analysis in the THB and introduce a technique to calculate the fluctuation power spectra of ne and Te. This paper is organized as follows: In Sec. II, we apply arithmetic operations on time series of emission intensity data and manipulate the ne and Te dependence.

MANIPULATING THE ELECTRON DENSITY AND TEMPERATURE DEPENDENCIES OF TIME SERIES

EXTRACTING POWER SPECTRAL DENSITIES FROM NOISY DATA

APPLICATION TO THE EXPERIMENTAL DATA

Findings

SUMMARY